This has been discussed in many threads.
BIGJOE was thinking about the old 1969-70 Boss 302 block.
The -NEW- Boss block does indeed have shorter cylinders. Stock 289/302 cylinders are between 5" and 5-1/8" long. They screwed up when they designed the new block and only made them 4.75" long. The 289/302 block already has some of the shortest cylinders out there, and shortening them like that has a good 1/4" of the piston pin hanging out of the bottom of the bore at BDC when a 3.4" (347) stroke is used. Therefore, the piston has very little support at BDC, rocks a lot, and can cause problems.
I believe Ford Racing is now pushing a 3.100" stroke 4.125" bore engine with this block that is supposed to measure around 331ci.
Buy a Dart Sportsman or a Dart SHP. Stay away from the defective Ford Boss 302 block.
Good Luck!